A class selector is a type of selector used to select and style elements with a specific class attribute. It is identified by the `.` symbol followed by the class attribute value.

```css
.warning {
  background-color: yellow;
  color: black;
  font-weight: bold;
}
```


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In this example, the class selector is `.warning`, which is used to select elements with a class attribute of warning. The style rules inside the curly braces will be applied to these elements.

Unlike `id` selectors, `class` selectors can select and style multiple elements on a single HTML page. This makes them very useful for creating reusable styles that can be applied to multiple elements. 

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In this example, both the `.warning` and `.error` selectors are `class` selectors. They make it easy to style multiple elements on the same page, allowing you to create different styles for different types of messages quickly.

You can also use the `class` selector and other selectors to create more specific rules. 
```css
#header .warning {
  font-style: italic;
}
```

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In this example, the `#header .warning` selector selects all the elements with a `class` attribute of warning that are descendants of an element with the id attribute of `header`. This allows you to apply styles to the elements with the " warning " class within the " header " element.

One potential downside of using `class` selectors is that they can make your CSS code more complex, as you may have to create and maintain multiple class selectors for different styles. However, the ability to reuse styles across multiple elements can make up for this complexity in many cases.

CSS, or Cascading Style Sheets, is a stylesheet language used to describe the presentation of a document written in a markup language like HTML. CSS is a fundamental building block of the modern web and controls the styling and layout of websites and web applications.

This section will cover the basics of CSS. You will learn the language's syntax, the most common CSS properties, and how to add CSS styles to your web page.

Selecting the elements you need becomes more challenging as your HTML code's complexity grows. This section will cover more advanced techniques of the selection.

This section will cover the general styling used for layout and text formatting to create a visually appealing and user-friendly design for your webpage. Part I.

Introduction to CSS Part I

Class Selectors